Coalesced synchronous Hindsight routing setting hooks into one serialized
session rebuild so cwd reloads and multi-setting updates cannot have
multiple continuations capture the same old state and leak fresh
session listeners.
Also re-read the current state after awaiting the previous queue flush
before installing the replacement state, so an unexpected concurrent
owner cannot leave the actual current state undisposed.
Fixes#1902
roboomp
a8b3aeaeaa